Talc is a hydrated magnesium silicate and is a naturally occurring mineral. Talc may be characterized as being either microcrystalline or macrocrystalline in nature. In particular, talc may generally be in the form of individual platelets. The individual platelet size of the talc (e.g., the median particle diameter as measured by the Sedigraph method) of an individual talc platelet (a few thousand elementary sheets) may vary from approximately 1 micron to over 100 microns, depending on the conditions of formation of the talc deposit. Generally speaking, microcrystalline talc has small crystals, which provide a compact, dense ore. Macrocrystalline talc has large crystals in papery layers. In a microcrystalline structure, talc elementary particles are composed of small plates as compared to macrocrystalline structures, which are composed of larger plates.
Talc may be used as a reinforcing additive for polymer compositions, such as polypropylene; however, microcrystalline talc is generally thought to be less reinforcing for this purpose. Two characteristics of a talc—its particle size distribution and its crude ore source—may affect the properties of talc-filled polypropylene molded articles. For example, talc particle size distribution affects the stiffness (or flexural modulus) and impact strength of a polypropylene molded article. It is shown that both stiffness and impact strength increases as talc particle size decreases. On the other hand, it has also been found that better long-term heat stability is generally realized with increased talc particle sizes.
The geographic source of talc affects the types and amounts of impurities in talc, as well as the crystalline structure. Talcs from certain geographic sources, such as Montana (Yellowstone and Beaverhead talcs, for example) and certain Australian talcs are microcrystalline. Whereas talcs from other locations are macrocrystalline, such as Vermont, Canada, and China talcs. It has historically been believed that microcrystalline talcs are inferior to macrocrystalline talcs as a reinforcing additive for polymers, particularly for polypropylene, and results in poor heat stability and unacceptable discoloration, regardless of the particle size of the talc used.
Microcrystalline talc, however, may be more readily available in certain geographic regions. Therefore, it may be desirable to provide talc compositions for use in stabilizing and strengthening polymers using microcrystalline talc. It may also be desirable to provide filler compositions containing microcrystalline talc that do not result in poor long term heat aging properties and do not result in unacceptable discoloration of the polymer.
The compositions and methods described herein may provide solutions to potential drawbacks with previously-existing compositions and/or methods.